chroma_sub8x8: Disable if chroma_2x2 is enabled

Since chroma_sub8x8 is a subset of chroma_2x2, both of these
experiments should not be turned on at the same time.

Change-Id: If54e0a3005d6fe26ade274700678558baf977f9c
diff --git a/configure b/configure
index 7e22e09..3f43b7e 100755
--- a/configure
+++ b/configure
@@ -522,6 +522,7 @@
     enabled compound_round && soft_enable convolve_round
     enabled smooth_hv && soft_enable alt_intra
     enabled intra_edge && enable_feature ext_intra
+    enabled chroma_2x2 && disable_feature chroma_sub8x8
 
     if ! enabled delta_q && enabled ext_delta_q; then
       log_echo "ext_delta_q requires delta_q, so disabling ext_delta_q"